Alzheimer's disease (AD) affects greater than 12 million aging people worldwide, and importantly, the number affected continues to grow. AD accounts for the majority of dementia clinically diagnosed after the age of 60. AD is generally characterized by the progressive decline of memory, reasoning, judgement and orientation. As the disease progresses, motor, sensory, and vocal abilities are affected until there is global impairment of multiple cognitive functions. The loss of cognitive function occurs gradually, typically leading to a diminished cognition of self, family and friends. Patients with severe cognitive impairment and/or diagnosed as end-stage AD are generally bedridden, incontinent, and dependent on custodial care. The AD patient eventually dies in about nine to ten years, on average, after initial diagnosis. Due to the incapacitating, generally humiliating and ultimately fatal effects of AD, there is a need to effectively treat AD upon diagnosis.
AD is characterized by two major physiological changes in the brain. The first change, beta amyloid plaque formation, supports the “amyloid cascade hypothesis” which conveys the thought that AD is caused by the formation of characteristic beta amyloid peptide (A-beta), or A-beta fragments thereof, deposits in the brain (commonly referred to as beta amyloid “plaques” or “plaque deposits”) and in cerebral blood vessels (beta amyloid angiopathy). A wealth of evidence suggests that beta-amyloid and accompanying amyloid plaque formation is central to the pathophysiology of AD and is likely to play an early role in this intractable neurodegenerative disorder. The second change in AD is the formation of intraneuronal tangles, consisting of an aggregate form of the protein tau. Besides being found in patients with AD, intraneuronal tangles are also found in other dementia-inducing disorders. Joachim et al., Alz. Dis. Assoc. Dis., 6:7-34 (1992).
Several lines of evidence indicate that progressive cerebral deposition of A-beta plays a seminal role in the pathogenisis of AD and can precede cognitive symptoms by years or even decades. Selkoe, Neuron, 6:487 (1991). Release of A-beta from neuronal cells grown in culture and the presence of A-beta in cerebrospinal fluid (CSF) of both normal individuals and AD patients has been demonstrated. Seubert et al., Nature, 359:325-327 (1992). Autopsies of AD patients have revealed large numbers of lesions comprising these 2 factors in areas of the human brain believed to be important for memory and cognition.
Smaller numbers of these lesions in a more restricted anatomical distribution are found in the brains of most aged humans who do not have clinical AD. Amyloid containing plaques and vascular amyloid angiopathy were also found in the brains of individuals with Down's Syndrome, Hereditary Cerebral Hemorrhage with Amyloidosis of the Dutch-type (HCHWA-D), and other neurodegenerative disorders.
It has been hypothesized that A-beta formation is a causative precursor or factor in the development of AD. More specifically, deposition of A-beta in areas of the brain responsible for cognitive factors is believed to be a major factor in the development of AD. Beta amyloid plaques are primarily composed of amyloid beta peptide (A-beta peptide). A-beta peptide is derived from the proteolytic cleavage of a large transmembrane amyloid precursor protein (APP), and is a peptide ranging in about 39-42 amino acid residues. A-beta 42 (42 amino acids long) is thought to be the major component of these plaque deposits in the brains of Alzheimer's Disease patients. Citron, Trends in Pharmacological Sciences, 25(2):92-97 (2004).
Similar plaques appear in some variants of Lewy body dementia and in inclusion body myositis, a muscle disease. Aβ also forms aggregates coating cerebral blood vessels in cerebral amyloid angiopathy. These plaques are composed of a tangle of regularly ordered fibrillar aggregates called amyloid fibers, a protein fold shared by other peptides such as prions associated with protein misfolding diseases. Research on laboratory rats suggest that the two-molecule, soluble form of the peptide is a causative agent in the development of Alzheimer's and that the two-molecule form is the smallest synaptotoxic species of soluble amyloid beta oligomer. Shankar, G. M., Nature Medicine (Jun. 22, 2008) online doi 10:1038 nm 1782.
Several aspartyl proteases, including beta-secretase and gamma-secretase, are thought to be involved in the processing or cleavage of APP, resulting in the formation of A-beta peptide. Beta secretase (BACE, also commonly referred to as memapsin) is thought to first cleave APP to generate two fragments: (1) a first N-terminus fragment (beta APP) and (2) a second C-99 fragment, which is subsequently cleaved by gamma secretase to generate the A-beta peptide. APP has also found to be cleaved by alpha-secretase to produce alpha-sAPP, a secreted form of APP that does not result in beta-amyloid plaque formation. This alternate pathway precludes the formation of A-beta peptide. A description of the proteolytic processing fragments of APP is found, for example, in U.S. Pat. Nos. 5,441,870, 5,712,130 and 5,942,400.
BACE is an aspartyl protease enzyme comprising 501 amino acids and responsible for processing APP at the beta-secretase specific cleavage site. BACE is present in two forms, BACE 1 and BACE 2, designated as such depending upon the specific cleavage site of APP. Beta secretase is described in Sinha et al., Nature, 402:537-554 (1999) (p510) and PCT application WO 2000/17369. It has been proposed that A-beta peptide accumulates as a result of APP processing by BACE. Moreover, in vivo processing of APP at the beta secretase cleavage site is thought to be a rate-limiting step in A-beta production. Sabbagh, M. et al., Alz. Dis. Rev. 3:1-19 (1997). Thus, inhibition of the BACE enzyme activity is desirable for the treatment of AD.
Studies have shown that the inhibition of BACE may be linked to the treatment of AD. The BACE enzyme is essential for the generation of beta-amyloid or A-beta. BACE knockout mice do not produce beta-amyloid and are free from Alzheimer's associated pathologies including neuronal loss and certain memory deficits. Cole, S. L., Vasser, R., Molecular Degeneration 2:22, 2007. When crossed with transgenic mice that over express APP, the progeny of BACE deficient mice show reduced amounts of A-beta in brain extracts as compares with control animals (Luo et al., Nature Neuroscience, 4:231-232 (2001)). The fact that BACE initiates the formation of beta-amyloid, and the observation that BACE levels are elevated in this disease provide direct and compelling reasons to develop therapies directed at BACE inhibition thus reducing beta-amyloid and its associated toxicities. To this end, inhibition of beta secretase activity and a corresponding reduction of A-beta in the brain should provide a therapeutic method for treating AD and other beta amyloid or plaque related disorders.
Consequently, the approach of regulating or reducing the formation of A-beta peptide formation and deposition as a potential treatment for AD has received tremendous attention and belief from both researchers and investors alike. A small molecule gamma-secretase inhibitor, LY450139 (“Semagacestat”), an A-beta lowering agent, is in phase II and Phase III clinical trials for the treatment of Alzheimer's Disease. The pharmacokinetics of semagacestat in plasma, as well as the plasma and cerebral spinal fluid (CSF) A-Beta peptide levels as pharmacodynamic responses to semagacestat administration were evaluated in healthy human subjects in single and multiple doses, and pharmacokinetic and pharmacodynamic changes were also assessed in mild to moderate AD patients in two (2) clinical trials (Expert Opin. Pharmacother. (2009), 10 (10); Clin. Neuropharmacol. 2007; 30 (pgs 317-325); and Neurology, 2006, 66 (pgs 602-624)).
Additional approaches have been taken in attempts to treat AD and plaque-related disorders. One such approach to reduce the formation of plaque on the brain involves the inhibition of and, therefore, the reduction of BACE activity. For example, each of the following PCT publications: WO 07/058602, WO 10/021680, WO 10/105179, WO 06/041404, WO 07/114771, WO 08/076045, WO 08/076046, WO 08/150217, WO 07/038271, WO 09/091016, WO 08/108378, WO 09/134617, WO 05/097767, WO 08/092785, WO 06/138265, WO 08/103351, WO 06/138230, WO 08/200445, WO 06/111370, WO 07/287692, WO 05/058311, EP 01942105, WO 08/133273, WO 08/133274, WO 07/049532, US20070027199, WO 07/038271, US20070072925, US20070203116, US20050282826, WO 08/118379, WO 06/076284, US20070004786, WO 06/083760, WO 07/011810, WO 07/011833, WO 08/054698 and WO10/128058, describe inhibitors of BACE, useful for treating AD and other beta-secretase mediated disorders.
Despite the many efforts and resources directed to researching A-beta lowering agents, there remains a need to identify safe and efficacious treatment agents for AD.